Varicella Proof of Immunity – 2015 Update

An Advisory Committee Statement (ACS) National Advisory Committee on Immunization (NACI)

Preamble

The National Advisory Committee on Immunization (NACI) provides the Public Health Agency of Canada (hereafter referred to as the Agency) with ongoing and timely medical, scientific, and public health advice relating to immunization. The Agency acknowledges that the advice and recommendations set out in this statement are based upon the best current available scientific knowledge and is disseminating this document for information purposes. NACI members and liaison members conduct themselves within the context of the Agency's Policy on Conflict of Interest, including yearly declaration of potential conflict of interest.

Table of Contents

Summary of Information Contained in this NACI Statement

The following summary provides key information for immunization providers. Please refer to the remainder of the Statement for details.

1. What

Varicella (chickenpox) is a viral disease that typically occurs in childhood.  Immunization is recommended as a part of the routine childhood schedule.

2. Who

For the targeted populations listed, the following conditions are considered criteria for immunity to varicella:

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Published: July 2016
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Related Topics

  1. Pregnant women exposed to varicella and Immunocompromised patients:
    Documented evidence of immunization with two doses of a varicella-containing vaccine; or laboratory evidence of immunity, except for hematopoietic stem cell transplant (HSCT) recipients who are considered susceptible, until appropriately revaccinated after transplant.
  2. Health Care Workers - Currently employed or who have been employed in another Canadian healthcare setting:
    Self-reported history or diagnosis of varicella or herpes zoster by a health care provider, if the disease occurred before the year of a one-dose varicella vaccine program implementation; documented evidence of immunization with two doses of a varicella-containing vaccine; or previous laboratory evidence of varicella immunity.  
  3. Health Care Workers - Newly hired into the Canadian healthcare system:
    Documented evidence of immunization with two doses of a varicella-containing vaccine; or previous laboratory evidence of varicella immunity.
  4. Healthy individuals:
    Self-reported history or diagnosis of varicella or herpes zoster by a health care provider, if the disease occurred before the year of varicella immunization program implementation; documented evidence of immunization with two doses of a varicella-containing vaccine; or previous laboratory evidence of varicella immunity.

3. How

Immunization should be offered to all susceptible individuals without contraindications to varicella vaccination. Pregnant women who are not immune to varicella should have vaccination offered post-partum.

New employment in a healthcare setting (i.e. Canadian HCW moving into a new facility within Canada) should be seen as an opportunity to assess immunity to varicella and to offer two doses of varicella vaccine when the status of immunity is in doubt.

4. Why

Identifying who is susceptible to varicella is important to allow for appropriate immunization strategies and post-exposure management.

I. Introduction

The National Advisory Committee on Immunization (NACI) currently recommends a 2-dose varicella vaccination schedule for all children aged 12 months to 12 years of age. Susceptible adolescents (12 to 17 years) and adults (18 years and older) should also be vaccinated. The Canadian Immunization GuideFootnote 1 enumerates criteria for proof of immunity. 

In the United States, the Advisory Committee on Immunization Practices (ACIP)Footnote 2  emphasizes limiting the number of false-positive reports of immunity by focusing on a diagnosis of varicella by a health-care provider, rather than self-reporting. In addition, because serologic evidence of varicella zoster virus (VZV) infection has been documented in 96%–97% of U.S.-born adults aged 20–29 years and in 97%–99% of adults aged ≥30 years tested during 1998–1999, birth in the United States before 1980 is considered evidence of immunity. This assumption does not apply to HCW, pregnant women, and immunocompromised persons. For these groups, certainty regarding immunity is necessary because of the possibility of nosocomial transmission to high-risk patients, transmission of the virus to the fetus, which might result in congenital varicella syndrome, and the possibility of severe disease.

The predictive value of self-reported disease was high in adults in the pre-vaccine eraFootnote 3,Footnote 4,Footnote 5. Since the introduction of universal vaccination programs in Canada, there has been a documented decrease in the prevalence of wild type varicellaFootnote 1. This decreased prevalence will lead to lower positive predictive value of self-reported history of chickenpox, therefore requiring a re-examination of the criteria used for the determination of varicella immunity. 

II. Methods

Following a search of the literature and critical appraisal of 22 individual studies that examined the validity of self-reported history of chickenpox among children and adolescents, adults and healthcare workers, the NACI Varicella Working Group (VWG) presented the evidence and proposed recommendations to NACI. Following thorough review of the evidence and consultation at NACI meetings (June 4 and October 2, 2014), the committee voted on specific recommendations. The description of relevant considerations, rationale for specific decisions, and knowledge gaps are described in the text of this update.

III. Epidemiology

Varicella occurs worldwide and, in temperate countries without vaccination programs, it is mainly a disease of childhood, developing in 50% of children by the age of 5 years and 90% by the age of 12 years. In Canada, since the introduction of childhood varicella immunization programs, studies have found a decrease in the disease burden. However, assessing the effect of varicella immunization programs on the incidence of the disease is difficult as varicella infections are significantly under-reported, with less than 10% of the expected cases reported annually. Nationally limited surveillance information on varicella is available through Canadian Notifiable Disease Surveillance System (CNDSS) and the Immunization Monitoring Program, ACTive (IMPACT). More information about current varicella epidemiology in Canada is available on the Agency website.

The predictive value of a reported history of varicella in determining natural immunity was assessed in 22 studies that looked at the seroprevalence and self-reported or parent-reported history in various populations, including large samples of children, teenagers and adults and healthcare workers, from temperate and tropical countries. Tables 1-3 summarize the positive and negative predictive values for the various populations.

Table 1 : Predictive value of reported history of varicella among children and youth (≤15 years)
Study Author(s) Year of study Country Age range Sample (N) PPV (%) NPV (%) Sensitivity (%) Specificity (%)
Parella DFootnote 6 2004-2006 USA 1-14 years 730 90 85 85 90
Field NFootnote 7 2012 UK 11-15 years 181 94 20 68 65
Bricks LFFootnote 8 2003-2006 Brazil 1-5 years 204 90 93 80 97
Diez-Domingo JFootnote 9 2003 Spain 6-15 years 184 95 48 86 74
Heininger UFootnote 10 1999-2000 Switzerland 1-18 years 449 98 87 90 97
Koturoglu GFootnote 11 2007-2008 Turkey 1-19 years 337 85 73 78 80
Mohanty S, Perella D, Jumaan A, et al.Footnote 12 2004-2006 USA 5-14 years 353 91 41 88 49
Kavaliotis JFootnote 13 1999-2001 Greece 1-14 years 632 88 89 81 93
Pooled       2136 92 81 84 91
Average         92 68 83 82
Table 2: Predictive value of reported history of varicella among youth and adults (≥15 years unless reported elsewhere) including HCWs
Study Author(s) Year of Study Country Age range Sample (N) PPV (%) NPV (%) Sensitivity (%) Specificity (%)
Parella DFootnote 6 2004-2006 USA 15-29 years 746 98.4 23.0 86.3 74.4
Alanen AFootnote 3 2000 16-45 years 558 98.5 13.2 85.3 63.2
Artega AFootnote 14 2006 Spain 18-26 years 226 86.2 25.0 64.3 53.7
Celikbas AFootnote 15 2005 Turkey 17-52 years 363 100.0 2.5 23.9 100.0
Dashraath PFootnote 16 2000-2005 Singapore 16-36 years 1987 87.2 83.2 94.3 67.1
Koturoglu GFootnote 11 2007-2008 Turkey >20 years 200 91.6 8.3 71.2 28.6
Abbas MFootnote 17 2006 Saudi Arabia 20-60 years 380 98.4 39.7 86.1 87.1
De Juanes JRFootnote 18 After 2000 Spain 23-40 years 76 98.2 30.0 79.7 85.7
Alp EFootnote 5 2010-2011 Turkey 19-60 years 1255 98.0 2.0 83.0 20.0
Trevisan AFootnote 19 2003-2005 Italy Mean 23 years 610 98.3 34.9 90.2 76.9
Almuneef MFootnote 4 2002-2003 Saudi Arabia 20-60 years 231 89.0 21.7 57.4 62.5
Kanra GFootnote 20 1998 Turkey 20-23 years 180 100.0 5.3 59.1 100.0
MacMahon EFootnote 21 2001-2002 UK Mean 27.5 years 623 95.0 18.1 77.1 55.8
Singru SFootnote 22 After 2000 India Mean 20 years 81 91.5 38.1 62.3 80.0
Mohanty SFootnote 12 2004-2006 USA 15-19 years 183 98.8 6.3 91.7 33.3
Pooled 8493 93.3 33.4 77.7 66.5
Average 95.3 23.4 74.1 65.9
Table 3: Predictive value of reported history of varicella among HCWs
Study Author(s) Year of study Country Age range Sample (N) PPV (%) NPV (%) Sensitivity (%) Specificity (%)
Abbas MFootnote 17 2006 Saudi Arabia 20 – 60 years 380 98 40 86 87
De Juanes JRFootnote 18 2003 Spain 23 – 40 years 76 98 30 80 86
Alp EFootnote 5 2010-2011 Turkey 19 – 60 years 1255 98 2 83 20
Trevisan AFootnote 19 2003-2005 Italy Mean 23 years 610 98 35 90 77
Almuneef MFootnote 4 2002-2003 Saudi Arabi 20 – 60 years 1058 89 22 57 63
Kanra GFootnote 20 1998 Turkey 20-23 years 180 100 5 59 100
MacMahon EFootnote 21 2001-2002 UK Mean 27.5 years 623 95 18 77 56
Singru SFootnote 22 Not reported India Mean 20 years 81 91 38 62 80
Pooled 4263 95 23 73 68
Average 96 24 74 71

The positive predictive value (PPV) of self- or parental reported disease was consistent among all age groups. In the healthcare worker population, there is a high pooled PPV associated with self-report of varicella disease of 95% (range: 89 - 100%). The same has been seen in youth over 15 years of age and adults (pooled PPV: 93.3% [range 86.2-100%] and in children and adolescents (pooled PPV: 92% [range: 85-98%]. These values are, however, dependent on disease prevalence.

To overcome the impact of changing disease prevalence, likelihood ratios (LR) of a positive test (or LR+: the impact of self-reporting a varicella disease on the probability of being immune to varicella) can be used. A LR+ is considered significant when greater or equal to 10 and means that the pre-test probability is significantly affected by the factor or test that is being studied. For varicella, the LR+ are low across all age groups. For the healthcare worker population, the LR+ (i.e. reporting a history of varicella disease) was low at 2.28, suggesting that a self-reported history does not improve significantly the pre-test probability of having had varicella in the past; the high PPV reported is inherently due to the high baseline seroprevalence of varicella.  Although this issue applies to the entire population, the impact of considering someone immune to varicella when they are susceptible has a greater impact in some populations, such as healthcare workers providing care to vulnerable patients.

IV. Vaccine Program Implementation

Table 4 provides information on the month and year of the one-dose varicella immunization program implementation in Canadian Provinces and Territories (P/T), after which reported history of varicella should not be considered as a proof of immunity, as its implementation had a marked impact on the prevalence of wild-type varicella disease.

Table 4: Implementation of one-dose varicella immunization programs in Canadian Provinces and Territories Footnote 23
Province or territory Initial implementation of a  varicella immunization program (month and year)
Prince Edward Island April 2000
Alberta March 2001
Northwest Territories Sept. 2001
Nova Scotia Jan . 2003
Nunavut Sept. 2002
Ontario Sept. 2004
New Brunswick Sept. 2004
Manitoba Oct. 2004
Newfoundland & Labrador Jan. 2005
Saskatchewan Jan. 2005
British Columbia Jan. 2005
Quebec Jan. 2006
Yukon Jan. 2007

V. Updated Recommendations

In formulating recommendations, the following observations have been taken into account:

  1. Due to the low incidence of disease following the implementation of immunization programs (one-dose program), self-reported history or clinical diagnosis by a health care provider is increasingly unreliable;
  2. History of varicella is going to be increasingly difficult to establish, including the ability of health care providers to diagnose cases without laboratory testing.
  3. Given the high baseline seroprevalence prior to the introduction of varicella programs, adults currently have a high likelihood of being immune to varicella;
  4. History of varicella infection as reported by a parent is of uncertain reliability. Both Canadian Pediatric Society (CPS) and ACIP accept only health care provider diagnosis as indicative of past infection with varicella.
  5. High-risk, susceptible individuals, including pregnant women, need to be protected from infection. Varicella during pregnancy can have adverse consequences for the mother and fetus, including congenital varicella syndrome.
  6. The value of serology in the absence of natural disease is uncertain, due to poor sensitivity of commercial assays in detecting vaccine-induced immunity. In contrast, vaccination is reliable and cost-effective.
  7. For the vast majority of vaccine-preventable diseases, NACI currently recommends that, when in doubt about previous disease or immunization status, an individual should be immunized.

It was also the shared opinion that the following principles should guide the determination of recommendations:

  • For individuals at high risk for varicella complications, self-reported history cannot be considered as acceptable evidence of immunity;
  • Because of the significant risk from varicella infection in immunocompromised individuals, it is important to ensure that HCWs are immune to varicella;
  • Serological testing is not practical and not necessary in the general population;
  • Immunizing individuals who lack adequate documentation is simple, safe, and effective.

Recommendation #1: For healthy individuals (including pregnant women without significant exposureFootnote a to VZV)

Individuals who have ANY of the following are considered immune to varicella:

  • Self-reported history or diagnosis of varicella or herpes zoster by a health care provider, if the disease occurred before the year of implementation of a one-dose vaccine program  (Grade B);
  • Documented evidence of immunization with two doses of a varicella-containing vaccine (Grade A);
  • Previous laboratory evidence of immunityFootnote b (Grade A);

Recommendation #2: For healthy pregnant women with significant exposureFootnote a to VZV and for immunocompromised individuals

Individuals who have ANY of the following are considered immune to varicella:

  • Documented evidence of immunization with two doses of a varicella-containing vaccine (Grade A);
  • Laboratory evidence of immunityFootnote b (Grade A);

Recipients of a hematopoietic stem cell transplant (HSCT) should be considered susceptible in the post-transplantation period regardless of a history of varicella disease or vaccination, or positive serologic test results, until appropriately revaccinated after HSCT.

Recommendation #3: For HCWs who are currently employed in or who have been employed in another Canadian healthcare setting

Individuals who have ANY of the following are considered immune to varicella:

  • Self-reported history or diagnosis of varicella or herpes zoster by a health care provider, if the disease occurred before the year of implementation of a varicella vaccine program  (one dose) (Grade B);
  • Documented evidence of immunization with two doses of a varicella-containing vaccine (Grade A);
  • Previous laboratory evidence of varicella immunityFootnote b (Grade A)

New employment in a healthcare setting (i.e. Canadian HCW moving into a new facility within Canada) should be seen as an opportunity to assess immunity to varicella and to offer two doses of varicella vaccine when the HCW has not been shown to be immune.

Following exposure to varicella within health care settings, verification of immunity, based on documented evidence of immunization with two doses of a varicella-containing vaccine or laboratory evidence of immunity, should be a part of post-exposure protocols.

Recommendation #4: For HCWs who are newly hired into the Canadian healthcare system

Individuals who have ANY of the following are considered immune to varicella:

  • Documented evidence of immunization with two doses of a varicella-containing vaccine (Grade A);
  • Previous laboratory evidence of varicella immunityFootnote b (Grade A).

Following exposure to varicella within health care settings, verification of immunity, based on documented evidence of immunization with two doses of a varicella-containing vaccine or laboratory evidence of immunity, should be a part of post-exposure protocols.

Recommendation #5: Immunization should be offered to all susceptible individuals without contraindications to varicella vaccination. Pregnant women who are not considered immune to varicella (as per Recommendation #1) should have vaccination offered post-partum (Grade A).

Detailed information about immunization in pregnancy and breastfeeding is available in the Canadian Immunization Guide.

Table 5. Levels of Evidence Based on Research Design
Level Description
I Evidence from randomized controlled trial(s).
II-1 Evidence from controlled trial(s) without randomization.
II-2 Evidence from cohort or case-control analytic studies, preferably from more than one centre or research group using clinical outcome measures of vaccine efficacy.
II-3 Evidence obtained from multiple time series with or without the intervention. Dramatic results in uncontrolled experiments (such as the results of the introduction of penicillin treatment in the 1940s) could also be regarded as this type of evidence.
III Opinions of respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees.
Table 6. Quality (internal validity) Rating of Evidence
Quality Rating Description

Table 6 Footnotes

Table 6 Footnote 1

General design specific criteria are outlined in Harris RP, Helfand M, Woolf SH, et al. Current methods of the US Preventive Services Task Force: a review of the process. Am J Prev Med 2001;20:21-35.

Return to table 6 footnote 1 referrer

Good A study (including meta-analyses or systematic reviews) that meets all design- specific criteriaTable 6 Footnote 1 well.
Fair A study (including meta-analyses or systematic reviews) that does not meet (or it is not clear that it meets) at least one design-specific criterionTable 6 Footnote 1 but has no known "fatal flaw".
Poor A study (including meta-analyses or systematic reviews) that has at least one design-specificTable 6 Footnote 1 "fatal flaw", or an accumulation of lesser flaws to the extent that the results of the study are not deemed able to inform recommendations.
Table 7. NACI Recommendation for Immunization - Grades
Grade Recommendation
A NACI concludes that there is good evidence to recommend immunization.
B NACI concludes that there is fair evidence to recommend immunization.
C NACI concludes that the existing evidence is conflicting and does not allow making a recommendation for or against immunization; however other factors may influence decision-making.
D NACI concludes that there is fair evidence to recommend against immunization.
E NACI concludes that there is good evidence to recommend against immunization.
I NACI concludes that there is insufficient evidence (in either quantity and/or quality) to make a recommendation, however other factors may influence decision-making.

List of Abbreviations

ACIP
Advisory Committee on Immunization Practices
CPS
Canadian Pediatric Society
HSCT
Hematopoietic Stem Cell Transplant
MMRV
Measles, mumps, rubella and varicella
NACI
National Advisory Committee on Immunization
NPV
Negative Predictive Value
PPV
Positive Predictive Value
Var
Varicella
VZV
Varicella zoster virus

Acknowledgments

NACI Members: Dr. I. Gemmill (Chair), Dr. C. Quach (Vice-Chair), Dr. N. Dayneka, Dr. S. Deeks, Dr. B. Henry, Ms. S. Marchant-Short, Dr. M. Salvadori, Dr. N. Sicard, Dr. W. Vaudry, Dr. D. Vinh, Dr. R. Warrington.

Former NACI Members: Dr. D. Kumar, Dr. B. Seifert.

Liaison Representatives: Dr. J. Blake (Society of Obstetricians and Gynaecologists of Canada), Dr. J. Brophy (Canadian Association for Immunization Research and Evaluation), Dr. A. Cohn (Centers for Disease Control and Prevention, United States), Dr. J. Emili (College of Family Physicians of Canada), Dr. M. Lavoie (Council of Chief Medical Officers of Health), Dr. C. Mah (Canadian Public Health Association), Dr. D. Moore (Canadian Paediatric Society), Dr. A. Pham-Huy (Association of Medical Microbiology and Infectious Disease Canada), Ms. E. Sartison (Canadian Immunization Committee).

Ex-Officio Representatives: Dr. (LCdr) K. Barnes (National Defence and the Canadian Armed Forces), Ms. G. Charos (Centre for Immunization and Respiratory Infectious Diseases [CIRID], Public Health Agency of Canada [PHAC]), Dr. G. Coleman (Biologics and Genetic Therapies Directorate, Health Canada [HC]), Dr. J. Gallivan (Marketed Health Products Directorate [MHPD], HC), Ms. J. Pennock (CIRID, PHAC), Dr. T. Wong (First Nations and Inuit Health Branch [FNIHB], HC).

Former Ex-Officio Representatives: Dr. J. Brooks (CIRID, PHAC), Dr. (LCol) P. Eagan (National Defence and the Canadian Armed Forces), Dr. B. Law (CIRID, PHAC), Ms. M. St-Laurent (CIRID, PHAC).

This statement was prepared by Dr. O. Baclic (CIRID, PHAC), Dr.R. Pless (CIRID, PHAC), and Dr. C. Quach (McGill University) and approved by NACI.

NACI also gratefully acknowledges the contribution of Dr. W. Ahmed (University of Ottawa).

References

Footnotes

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